Tom, Other than walking one leg of the ODP - Horseshoe Pass to Llanarmon yn ial, which was superb - it is not a path I know much about. However, the Ramblers handbook - Walk Britain - lists Sherpa as the only baggage carriers and one web site which may be of use,
They also offer a telephone number for the Offas Dyke Association - 01547 528753.
Good Luck!